env/solaris-amd64/joyent: update README to add gcc
env/solaris-amd64/joyent: update README to add gcc

Change-Id: I5ed1f395b4182c57e8877d59b1feb4462db25808
Reviewed-on: https://go-review.googlesource.com/22743
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build